Made of stainless steel. No 4 suits scalpel blade sizes : 20, 21, 22, 23 and 24. No 3 suits scalpel blade sizes: 10, 11, 12 and 15. The folding handles are a very handy little invention that allows the blades to be folded back into the handle protecting the blade and allowing safer storage and handling. The wider hand grip is also more comfortable to hold.